Lactic acid is broadly used in chemical raw materials such as of medical treatments, and cosmetics. Also, polylactic acid which is obtained by using lactic acid is drawing attention as a biodegradable plastic which is degraded finally into carbon dioxide and water by a microorganism and the like. Because of this, it is necessary to produce lactic acid with a low cost and a high productivity.
As the method for producing lactic acid, there is known a biological method in which it is produced by fermenting a sugar by a lactic acid bacterium. However, since lactic acid bacteria have low acid resistance, in order to obtain a high productivity by this method, it is necessary to convert the lactic acid produced by the fermentation into a lactic acid salt by neutralizing it with an alkali. Since such neutralization with an alkali requires a step for restoring lactic acid from the lactic acid salt, the production process becomes complex and the production cost also becomes high.
Accordingly, as the method for obtaining lactic acid without carrying out neutralization with an alkali, there have been shown a method which uses a transformant containing an acid resistant microorganism such as a yeast belonging to the genus Saccharomyces as a host and prepared by introducing a gene encoding a lactate dehydrogenase into the acid resistant microorganism (Patent Reference 1); and a method which uses Saccharomyces cerevisiae (budding yeast) into which a gene encoding a lactate dehydrogenase was introduced and in which a gene encoding a pyruvate decarboxylase 1 was deleted or inactivated (Patent Reference 2).